1. Field of the Invention
This invention relates to emergency position indicating radio beacons (EPIRBs) which are used in search and rescue to locate people, such as those lost at sea, shipwrecked, or stranded in the wilderness, through the transmission of radio signals and information on two different guard channel frequencies, one frequency being double the frequency of the other, and in particular, to a radio beacon that transmits RF information at two guard frequencies generated by improved frequency multiplier circuitry resulting in a more compact housing with higher signal gain, improved efficiency, decreased distortion, and decreased power dissipation, allowing for operation with less batteries.
2. Description of the Prior Art
Emergency position indicating radio beacons (EPIRBs) are used by people who are shipwrecked, lost, or stranded at sea for transmitting distress information signals at frequencies monitored on guard channels by the Coast Guard and other search and rescue forces. An EPIRB is a small, portable, battery-powered RF transmitter having precoded messages of emergency identification that is stored in an emergency equipment environment such as a raft, aboard large ships or aircraft, activated usually manually or possibly by contact with water. Volumetric size and weight are very important factors in the effective storage of EPIRBs when not in use. Storage design is balanced against RF signal output power and strength for effective signal radiation and transmission, a critical design factor. The transmitted signal allows rescue teams to locate and home in on a lost boater or pilot's position. Guard or RF emergency channels are monitored radio frequencies of 121.5 MHz and 243.0 MHz so as to receive distress calls in the form of swept audio tones or pulsed Mayday, S.O.S. and other emergency data. The frequency, 121.5 MHz, was the original channel that distress signals were first transmitted and received on, while 243.0 MHz has become more popular because of its increased range, antenna efficiency and usage by the military. Although equipment exists which operates at both frequencies, older models still in use receive distress calls only at 121.5 MHz. Accordingly, contemporary EPIRBs are being designed to transmit at both guard channel frequencies so as to more effectively communicate with entities such as the U.S. Coast Guard ships and facilities, rescue planes and ships at sea.
In order to transmit two distinct RF frequencies from a single EPIRB, dual frequency radio beacons known in the art typically generate a first single frequency at 121.5 MHZ which is doubled in a conventional frequency doubling channel multiplier to produce the other signal at 243.0 MHz. The first generated signal is fed on one branch into the doubling multiplier and also to a different branch that bypasses the multiplier so that both output signals, 121.5 MHz and 243 MHz, may be broadcast by a combining network. However, the conventional frequency multiplier employed in these radio beacons generate undesired harmonics of increasing order in a Fourier series, causing unwanted power dissipation and wave distortion. For a particular power output, a large number of batteries is required and hence, EPIRB housing size has been larger than desired. The EPIRB should be lightweight and take up as little space as possible without sacrificing power output.
The frequency multipliers employed in the past include non-linear amplifiers which generate harmonics in their output and have a tuned load that resonates at one of the harmonics, and diode multipliers. The non-linear harmonic generating amplifier doubles frequency by driving the input of two transistors with opposite polarities usually obtained from opposite ends of a transformer or tank circuit. The problem with the harmonic generating amplifier, however, is that the output produces the higher order harmonics including odd harmonics which tend to dissipate power and decrease gain. Meanwhile, the diode multipliers also realize power loss, in the order of 6 db, and the input may be as much as four times the output. In addition, the efficiency of the diode multiplier is relatively low.
In short, both type EPIRB frequency doublers circuitry necessitate larger power supplies to satisfy increased power requirements because of the power dissipation, wave distortion and low efficiency caused by the presence of odd harmonics. This makes the beacons heavier and more expensive to operate. Moreover, since it is often the case that lost or stranded personnel are unsure of when their rescue will be made or since days may lapse before a group is discovered, the problem of power dissipation could be detrimental. A beacon which uses half the power of conventional beacons would significantly decrease the battery drain, require smaller battery packs and allow additional or spare batteries to be carried. Consequently, there exist a need for a radio beacon with a more efficient frequency multiplier capable of reducing the power requirements, increasing the Q factor of the circuit and improving the overall gain.
The present invention solves the problems discussed above by providing a radio beacon with a frequency doubler circuit that is able to suppress extraneous odd RF signal harmonics and inherently rectify the signal to increase the quality factor of the circuit and minimize the power dissipation of the battery source so that only half the conventional battery size is required to achieve the required power output. The present invention accomplishes these goals by incorporating a push-push amplifier circuit that utilizes two base grounded transistors with a high Q input phase splitter that is able to multiply the incoming source signal while eliminating the odd harmonics of the signal to minimize power dissipation.